1. Field of the Invention
The present invention is directed to a method for operating a magnetic resonance tomography device and to a magnetic resonance tomography apparatus for implementing the method.
2. Description of the Prior Art
The requirements for a gradient coil are mainly dependent on the pulse sequence that is applied for MR-imaging. In the following, "conventional" pulse sequences are differentiated from "fast" pulse sequences. As used herein, "conventional" pulse sequences can be characterized by only one nuclear magnetic resonance signal being selected per excitation. Examples of conventional pulse sequences are traditional spin echo methods or gradient echo methods for example, but also the faster methods such as FLASH (described in U.S. Pat. No. 4,707,658) and FISP (described in the U.S. Pat. No. 4,769,603) are examples thereof. As used herein, "fast" MR-imaging techniques are those wherein a large number of nuclear magnetic resonance signals are selected subsequent to an excitation. The EPI method (described in the U.S. Pat. No. 4,165,479), in particular, is such a sequence, as well as the turbo spin echo method, the GRASE method and the HASTE methods.
A high image quality and a large measuring volume are particularly important given conventional pulse sequences. The following requirements with respect to the gradient coils thereby result: Large linearity volume (.apprxeq.5% linearity in the linearity volume of 40-50 cm), moderate gradient intensities (10-20 mT/m) and moderate switching times (.apprxeq.1 ms).
Great value is particularly placed on the speed in fast pulse sequences, but compromises must be made regarding other parameters. High gradients (20-40 mT/m) must be switched or activated in a very fast manner; this is a specific requirement with respect to the gradient system (switching times approximately 100-500 .mu.s). Due to the necessary high amplitude change rates of the magnet fields, currents are induced in a patient to be examined, which currents can lead to peripheral muscle stimulations. The stimulation is particularly determined by the maximum field boost. With the given requirements with respect to the gradient intensity and the switching time, the field boost and thus the stimulation risk can only be reduced by reducing the linearity volume of the gradient coil.
To that end, it is known from the German OS 195 40 746 to use a modular gradient coil system. A central modular coil alone is utilized for fast pulse sequences. This central modular coil exhibits only a relatively small linearity volume. Since the efficiency of a coil is approximately proportional to the volume, high gradient intensities and short rise times can be realized with such a coil. Although the measuring volume is limited, fast pulse sequences can be realized, the field boost remains limited. Given operation of the MR-tomography device with conventional sequences, correcting coils are connected to the central part of the coil, these correcting coils increasing the linearity volume; however, this is at the expense of the gradient performance.
Only prior to the measurement, the user has the choice to either optimize the gradient coil system for maximal gradient intensity/gradient rise speed or to optimize it for a maximal linearity volume given this modular gradient coil system.